A fragment of the Pacific tectonic plate beneath Tokyo may be the source of recurring large earthquakes in this region, according to research published online this week in Nature Geoscience. The fragment is jammed between the various tectonic plates that interact there, increasing friction between the plates and setting the stage for earthquakes.

Shinji Toda and colleagues found the discrete fragment by analysing the location and distribution of hundreds of thousands of past earthquakes that have occurred beneath central Japan. They suggest that the fragment broke off the Pacific plate as a result of the resistance of large submarine volcanoes that were present on the plate as it subducted.

The identification of this fragment further complicates the tectonic picture of central Japan, which sits at the boundary of several tectonic plates.